Cell Bio exam 2

Cell Membrane

A lipid barrier that surrounds and protects the inside of the cell. It is semi-permeable, controls/allowing some molecules to pass through but blocking others. Follows fluid mosaic model. mainly made up of lipids and proteins

Fluid vs Mosaic

Molecules (lipids and proteins) move around within the membrane vs membrane is made of many different molecules (lipids, carbs, proteins) like a ___ art piece

Lipids

Consist of a hydrophilic head (likes water) made of a phosphate group (polar) and a hydrophobic tail (doesn’t like water) made of fatty acids (non-polar)

Liposome

phospholipids form this in water, their hydrophobic tails hide from water while hydrophilic heads face the water forming a spherical structure. It is energetically favorable because it reduces the exposure of hydrophobic tails to water making it more stable and requiring less energy to maintain structure.

Membrane proteins

Receptors (receive signals), channels (let things in and out of the cell), adhesion molecules (help cells stick together)

Water Molecules and Hydrogen Bonding

Water molecules prefer to bond with each other (requires less energy and is more stable), so they "push" hydrophobic molecules together to avoid breaking these bonds, leading to the formation of stable membranes.

To avoid disrupting water, lipids self-organize into structures like:

Bilayers- found in cell membranes

Micelles- tiny spheres with tails inside

liposomes- spheres with an inner water space

asymmetrical structure

Membranes are not identical on both sides,

How is ___ created?

1. Membrane synthesis starts in the Endoplasmic Reticulum (ER).

2. Phospholipids are added to one side (cytosolic face).

3. Some are flipped to the other side using flippases (special enzymes).

4. Glycolipids (lipids with carbs) stay on the non-cytosolic side

Non-cytosolic

• ___ means facing away from the cytoplasm, whether that’s outside the cell OR inside an organelle. "Exterior" only makes sense for the cell membrane, but not for membranes inside the cell. Because membranes move around, calling them "___" is more precise and always correct.

Phosphatidylserin

___ is normally found on the inner (cytosolic) side, unless flipped during apoptosis (cell death). of the membrane

Membrane fluidity

Membranes aren’t fixed to one shape they are flexible and more like a liquid, this allows molecules to move around so proteins can interact and do their jobs. allows cells to change shape for things like cell division and allows membranes to self repair if damaged.

What Affects Membrane Fluidity?

Temperature- too cold causes the membrane to become rigid like butter in the fridge. Too hot causes the membrane to become fluid.

Fatty acid tails- unsaturated fats (with kinks) makes the membrane more fluid (like olive oil) saturated fats (straight tails) make the membrane less fluid.

Cholesterol-n acts like a temp buffer, at high temps it keeps the membrane from getting too fluid, and at low temps it keeps the membrane from becoming too rigid

What happens if a membrane is cooled or heated to extreme temperatures?

If ___too much → The membrane becomes solid, and the cell may die.

If ___ too much → The membrane becomes too leaky, and the cell loses control of what enters and exits.

membrane proteins

Receptors → Receive signals (e.g., hormone receptors)
Adhesion molecules → Help cells stick together Channels & Pumps → Move substances in and out of the cell
Transporters → Help large molecules cross the membrane

Proteins help move important molecules like nutrients, ions, and waste.

Carbs in membrane

____ attach to lipids (glycolipids) or proteins (glycoproteins). ___ help membrane by: ✔ Cell recognition – Cells use carbohydrates to identify each other (like ID tags).
✔ Cell signaling – Help cells communicate.
✔ Adhesion – Help cells stick together.
✔ Protection – Some carbohydrate layers make the cell “slimy” (like mucus)

A slimy carbohydrate layer protects the cell from damage (like in the stomach or lungs).

It also helps prevent bacteria from sticking to the cell.

Integral Membrane Proteins (embedded in the membrane)

Transmembrane proteins → Pass through the entire membrane.

Monolayer-associated → Attached to only one side of the membrane.

Lipid-linked → Anchored to a lipid in the membrane.

Peripheral Membrane Proteins

attached to other proteins, not embedded. a protein will associate with the membrane. If it has hydrophobic regions, it will likely be inside the membrane. If it only interacts with other proteins, it will stay outside the membrane.

Alpha helices

a protein forms this structure inside the membrane. Common in transmembrane proteins. They allow proteins to be flexible and functional.

Beta-barrels

a protein forms this structure inside the membrane. Form pores (holes) in the membrane to allow molecules through. Can be used to from “barrels”

Bacteriorhodopsin

This is a membrane protein found in some bacteria. It helps pump protons (H⁺ ions) across the membrane using light energy.

Aquaporins (Water channel proteins)

Some membrane proteins act as channels to allow molecules to pass through ____ being one. They are membrane proteins that allow water molecules to pass in and out of the cell quickly. Without __, water movement would be too slow. They are selective, meaning only water can pass through—other molecules are blocked.

What Gives the Cell Membrane Strength?

The Cell Cortex → A network of proteins on the cytoplasmic side of the membrane.

Example: Spectrin → A protein that gives red blood cells their shape and flexibility.

If a cell membrane is too weak Cells could easily break or lose their shape, leading to diseases like hereditary elliptocytosis (HE), where red blood cells become misshapen.

Passive Transport

No Energy Required. Molecules move down their concentration gradient (from high to low concentration). Includes simple diffusion and facilitated diffusion

Simple diffusion

Small, nonpolar molecules (O₂, CO₂) pass directly through the membrane.

Facilitated Diffusion

Transport proteins help larger or charged molecules cross the membrane (e.g., glucose transporters, ion channels). Two types of transport proteins:

1. Channel Proteins – Create a pore (e.g., ion channels for Na⁺, K⁺).

2. Carrier Proteins – Bind and change shape to transport molecules (e.g., glucose transporters).

Active Transport

(Requires Energy) Moves molecules against their concentration gradient (from low to high concentration). Requires ATP or another energy source. Example: The Na⁺/K⁺ pump, moves sodium and potassium ions to maintain cell potential. Important for Maintaining ion gradients (e.g., Na⁺/K⁺ pump). Absorbing nutrients even when the concentration is low outside the cell.

Ion channels

proteins that allow specific ions to pass through the membrane. They are selective, meaning each channel is specific for a particular ion (e.g., Na⁺ channels only allow sodium).

Voltage-gated channels

Open/close based on electrical charge differences across the membrane (important in nerve signaling).

Ligand-gated channels

Open when a molecule (ligand) binds to the channel.

Mechanically-gated channels

Open in response to physical forces like pressure (e.g., touch receptors in skin).

Osmosis

the movement of water across a membrane from low solute concentration to high solute concentration. Water moves through aquaporins, special channels that allow rapid water flow.

Tonicity

describes how solutions affect cell water movement such as Hypotonic Solution – Water enters the cell, causing it to swell or burst. Hypertonic Solution – Water leaves the cell, causing it to shrink. Isotonic Solution –No net water movement; the cell remains the same.

Coupled Transport

Uses the energy from one molecule moving down its gradient to power the movement of another molecule against its gradient. Types of coupled transporters: Symport – Both molecules move in the same direction (e.g., Na⁺-glucose symporter). Antiport – Molecules move in opposite directions (e.g., Na⁺/H⁺ exchanger).

Action potential

A rapid change in membrane potential (voltage due to ion movement) that allows nerve cells (neurons) to send signals. It happens when a neuron receives a strong enough stimulus to cause depolarization (membrane becomes less negative)

Voltage-gated Na⁺ channels

Open during nerve impulses, allowing Na⁺ to rush in. involved in action potential. This depolarizes the membrane (inside becomes more positive).

Voltage-gated K⁺ channels

Restore the resting potential by allowing K⁺ to exit. involved in action potential

Na⁺/K⁺ pump

Resets ion gradients after a signal is sent. involved in action potential

Calcium Pumps

Controlling Intracellular Ca²⁺, Ca²⁺ must be kept at low levels inside the cell to prevent unwanted signaling. Pumps Actively remove Ca²⁺ from the cytoplasm into the ER, SR (sarcoplasmic reticulum), or out of the cell. Example: The Sarcoplasmic Reticulum Ca²⁺ Pump in muscle cells helps control muscle contractions.

Why is Ca²⁺ Regulation Important?

• Muscle contraction: Ca²⁺ release triggers contraction; Ca²⁺ removal relaxes muscles.

• Neurotransmitter release: Ca²⁺ signals neurons to release chemical messages.

• Cell signaling: Ca²⁺ is involved in many cellular processes, including enzyme activation.

Voltage-sensing ion channels

Respond to changes in membrane voltage by movement of a voltage sensor

Stress-gated ion channels

ion channels can be opened by mechanical stress Example: hair cells in inner ear

Aquaporins

are specialized channel proteins that allow water molecules to move rapidly across the cell membrane. Unlike simple diffusion, aquaporins significantly increase water permeability, ensuring efficient water balance in cells.

How Do Cells Deal with Osmotic Shock?

Osmotic shock occurs when there is a sudden change in the solute concentration around a cell, causing excessive water movement into or out of the cell. Cells deal with osmotic shock by regulating water and solute balance. They use special channels, like aquaporins, to control water flow, and structures like the cell wall (in plants) or contractile vacuoles (in some organisms) to prevent damage from too much water entering or leaving.

Leak Channels (Passive, Always Open)

Allow a constant flow of ions to maintain resting membrane potential.

Example: K⁺ leak channels help establish the cell’s negative charge inside

Gated Ion Channels (Open/Close in Response to Stimuli)

Voltage-Gated Channels: Open when the membrane potential changes (important in nerve signaling).

Ligand-Gated Channels: Open when a molecule (ligand) binds to them.

Mechanically-Gated Channels: Open in response to physical forces like pressure or stretch.

Membrane potential

the electrical charge difference across a cell’s membrane.

It is created by unequal distribution of ions (Na⁺, K⁺, Cl⁻) inside and outside the cell.

The inside of the cell is usually negative compared to the outside.

How is Membrane Potential Maintained?

Na⁺/K⁺ Pump (Sodium-Potassium Pump) Uses ATP to move 3 Na⁺ out and 2 K⁺ in. Helps keep more Na⁺ outside and more K⁺ inside, creating an electrical difference.

K⁺ Leak Channels Always open, allowing K⁺ to slowly leave the cell. This makes the inside of the cell more negative.

Other Ion Channels and Transporters Voltage-gated Na⁺ and K⁺ channels open and close to generate nerve signals. Cl⁻ channels help balance the charge inside the cell.

Why Are Action Potentials Important?

Essential for brain function and communication between neurons.

Controls muscle movements, including heartbeats.

Allows for reflexes and sensory processing (touch, pain, vision).

How Do Action Potentials Travel?

Once an action potential starts, it spreads along the neuron like a wave.

This process is called propagation and ensures the nerve signal reaches its target (another neuron or muscle).

Synaptic Transmission

How nerve signals pass from one neuron to another or to a muscle.

The connection point is called a synapse, which can be:

Electrical Synapse (direct ion flow, rare in humans).

Chemical Synapse (uses neurotransmitters, most common).

Carbohydrates

molecules made of carbon (C), hydrogen (H), and oxygen (O) in the general formula (CH₂O)n. They are essential energy sources for cells.

types:

Monosaccharides (Simple Sugars) – Single sugar molecules (e.g., glucose, fructose).

Disaccharides – Two monosaccharides linked together (e.g., sucrose, lactose).

Polysaccharides – Long chains of monosaccharides used for energy storage or structure: Glycogen (in animals) and starch (in plants) store energy. Cellulose (in plants) provides structural support.

Regulation of Glucose Storage and Use

Hormones control blood sugar levels:

Insulin (from the pancreas) lowers blood sugar by helping cells absorb glucose.

Glucagon (also from the pancreas) raises blood sugar by triggering glycogen breakdown.

Epinephrine (adrenaline) also raises blood sugar for energy during stress.

Cellular respiration

The process of extracting energy from food. Compounds are broken down (oxidized) and energy is being released

Storage Polysaccharides

Starch (in plants) and Glycogen (in animals) are made of repeating glucose units. They store energy for later use. Starch is stored in granules in plants.

Excitatory Neurotransmitters (Increase Activity)

Glutamate → Most common excitatory neurotransmitter; important for learning and memory.

Acetylcholine (ACh) → Activates muscles; involved in learning and attention.

Inhibitory Neurotransmitters (Reduce Activity)

GABA (Gamma-Aminobutyric Acid) → Prevents excessive nerve activity; helps with relaxation and sleep.

Glycine → Mostly found in the spinal cord; helps control reflexes

Modulatory Neurotransmitters (Affect Multiple Systems)

Dopamine → Controls pleasure, motivation, and movement (low levels linked to Parkinson’s).

Serotonin → Regulates mood, sleep, and appetite (low levels linked to depression).

Norepinephrine (Noradrenaline) → Increases alertness and focus; part of the "fight-or-flight" response.

What are the two forces that drive the movement of a charged solute across a membrane?

Concentration Gradient – Ions move from high to low concentration (like a ball rolling downhill).

Membrane Potential – The inside of the cell is usually negatively charged, so positively charged ions (e.g., Na⁺, K⁺, Ca²⁺) are attracted inside, while negatively charged ions (e.g., Cl⁻) are repelled.

Electrochemical Gradient

1. Concentration Gradient → Ions move from high to low concentration.

2. Membrane Potential → Opposite charges attract (positive ions move inside the cell).

This gradient determines the direction and speed of ion movement across the membrane.

Structural Polysaccharides

Cellulose is a structural molecule in plant cell walls. It provides support and strength to plants.

The Four Stages of Cellular Respiration

Stage 1: Breakdown of Large Molecules

Before cells can use food for energy, macromolecules (like starch, proteins, and fats) must be broken down. Happens outside the cell and in the digestive system. The main goal convert complex molecules into simpler units that cells can absorb

Stage 2: Glycolysis (Happens in the Cytoplasm)

Stage 3: The Krebs Cycle (Citric Acid Cycle) in the Mitochondria

Stage 4: Electron Transport Chain (ETC) & Oxidative Phosphorylation

Glycolysis

The process of breaking down glucose (C₆H₁₂O₆) into pyruvate to produce ATP (energy). Occurs in The cytoplasm of the cell. Can occur with or without oxygen). 1 glucose (C₆H₁₂O₆) → 2 pyruvate + 2 ATP + 2 NADH. If oxygen is available, Pyruvate goes into the Krebs cycle for further breakdown. If no oxygen is available, Pyruvate is converted into lactic acid (in animals) or ethanol (in yeast).

Glycogenesis

The process of converting glucose into glycogen (a storage form of glucose). Occurs in the liver and muscles. Excess glucose (from food) is linked together into glycogen chains for storage. This happens when blood sugars are high. The hormone insulin stimulates ____. Used to prevent excess glucose from building up in the blood and provides a stored energy source.

Glycogenolysis

The process of breaking down glycogen into glucose when the body needs energy. Occurs in the liver and muscles. Glycogen is broken down into individual glucose molecules. This happens when blood sugar levels are low. The hormone glucagon (from the pancreas) stimulates glycogenolysis. Keeps blood sugar levels stable when you haven’t eaten.

Gluconeogenesis

The process of making glucose from non-carbohydrate sources, like proteins or fats. Occurs in the liver (mainly) and the kidneys. The body converts amino acids (from proteins) or glycerol (from fats) into glucose. This happens during starvation, fasting, or intense exercise when glycogen stores are empty. The hormone glucagon (from the pancreas) stimulates gluconeogenesis. Ensures the body has glucose even when food isn’t available.

Fermentation

Anaerobic (without oxygen) process that allows cells to keep producing ATP when oxygen is unavailable. It occurs in the cytoplasm, not the mitochondria. Produces only 2 ATP per glucose, compared to the ~36 ATP produced in aerobic respiration. Helps regenerate NAD⁺, which is needed for glycolysis to continue.

Lactic Acid Fermentation

Humans & Bacteria. Occurs in muscle cells (during intense exercise) and some bacteria (used in yogurt & cheese production). Process Glucose → Pyruvate → Lactic Acid. This process regenerates NAD⁺, allowing glycolysis to continue. Causes muscle fatigue and soreness due to lactic acid buildup. When oxygen is restored, lactic acid is converted back into pyruvate and enters aerobic respiration.

Alcoholic Fermentation

Yeast & Some Bacteria. Occurs in Yeast and some bacteria. Process Glucose → Pyruvate → Ethanol + CO₂. Regenerates NAD⁺ to keep glycolysis going. Used in making beer, wine, and bread (the CO₂ makes bread rise). The ethanol produced in this process is the alcohol found in alcoholic beverages.

Pyruvate Oxidation

After glycolysis, we have 2 pyruvate molecules (from 1 glucose). If oxygen is present, pyruvate moves into the mitochondria for further processing. Pyruvate oxidation prepares pyruvate for the Krebs cycle by converting it into acetyl-CoA.

Pyruvate Dehydrogenase Complex (PDC)

Once glycolysis is done, we have pyruvate, a 3-carbon molecule. But for the Krebs cycle (Citric Acid Cycle) to begin, pyruvate must first be converted into acetyl-CoA. This happens in a large enzyme system called the PDC, which works like a processing plant.

Krebs Cycle

acetyl-CoA enters the Krebs cycle, which happens inside the mitochondrial matrix. The goal of this cycle is to break down acetyl-CoA completely into CO₂, while capturing high-energy electrons in NADH and FADH₂.

Phosphofructokinase (PFK)

an enzyme that controls the speed of glycolysis by regulating an important step, It converts fructose-6-phosphate into fructose-1,6-bisphosphate, which is a committed step in glycolysis. Since this step requires ATP, the cell only wants to do it when energy is needed.